1. Field of the Invention
The present invention generally relates to forward bias beta degradation in bipolar transistors and, more particularly, to an emitter contact structure for inhibiting such degradation when attributable to electromigration of the contact metallurgy.
2. Description of the Prior Art
It is generally well understood that long aluminum stripes, customarily used in making contact to bipolar and other semiconductor devices have relatively short lifetimes, due to electromigration, as compared to shorter stripes. Segmented stripes to increase electromigration resistance by reducing solute concentration depletion and by reducing extrusion pressure are disclosed in Invention Disclosure Bulletins on segmented stripe devices by N. G. Ainslie et al, IBM.RTM. Technical Disclosure Bulletin dated December 1970, pp. 2011 and 2014, respectively. However, the electromigration impact on the semiconductor devices, which are connected to the metallurgy subjected to electromigration, is not addressed. Similar findings as to the relationship between the amount of electromigration and the length of the conductor being subjected to electromigration forces are discussed in the paper "Threshold Current Density and Incubation Time to Electromigration in Gold Films" by E. Kinsbron et al, Thin Solid Films, 46 (1977), pp. 139-150.
Stress gradients, in metallurgy subjected to electromigration, oppose electromigration according to the analyses given in the papers "Stress Generation by Electromigration" by I. A. Blech et al, Applied Physics Letters, Vol. 29, No. 3, Aug. 1, 1976, pp. 131-133 and "Measurement of Stress Gradients Generated by Electromigration" By I. A. Blech et al, Applied Physics Letters, Vol. 30, No. 8, Apr. 15, 1977, pp. 387-389.
It is also known that electromigration of contact metallurgy causes a stress build-up at the emitter contact of a bipolar transistor with a consequent reduction in the current amplification factor (.beta.) of the transistor. This is discussed in the paper "The Relationship Among Electromigration, Passivation Thickness, and Common-Emitter Current Gain Degradation Within Shallow Junction NPN Bipolar Transistors" by R. S. Hemmert et al, Journal of Applied Physics, Vol. 53, No 6, pp. 4456-4462, 1982.